1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus, and more particularly to an ultrasound diagnostic apparatus which can be used for evaluation of dynamic characteristics of a diagnosed bone.
2. Description of the Related Art
Easy quantitative measurement of mechanical characteristics such as bone strength is desired for diagnosing bone metabolic diseases such as osteoporosis, judging fracture risk, and quantitatively diagnosing bone union after treatment of bone fracture.
The evaluation of bone formation and bone union depends largely on X-ray photography, but quantitatively diagnosing bone strength by means of X-ray photography is very difficult. The X-ray irradiation may also have adverse effects on a measurement target.
To evaluate dynamic characteristics of a bone, the ultrasound diagnostic apparatus discussed in Japanese Patent Laid-Open Publication No. 2004-298205 forms ultrasonic beams on the bone (i.e., measurement target), obtains echo signals corresponding to the individual ultrasonic beams to specify surface points corresponding to a bone surface for respective echo signals, and generates shape data of the bone surface based on the surface points obtained from the echo signals. Moreover, the apparatus calculates a distortion of the bone subjected to an external load based on the obtained shape data, and uses the obtained distortion value as a reference value in the measurement of bone strength, or determination of cured state of a fractured bone. The distortion obtained in this manner is a numerical value that can be used as an evaluation reference. Thus, the ultrasound diagnostic apparatus can realize a quantitative measurement of bone strength based on the obtained distortion.
However, the ultrasound diagnostic apparatus discussed in Japanese Patent Laid-Open Publication No. 2004-298205 calculates a distortion representing a position change rate in the radial direction of a diagnosed bone. More specifically, the apparatus obtains a distortion ε defined by a formula ε=Δd/L when L represents the length of a measurement range and Δd represents a maximum displacement (i.e., maximum deflection) of a diagnosed bone in the radial direction when an external load is applied thereon. Namely, according to the Japanese Patent Laid-Open Publication No. 2004-298205, any factors other than a shape change on the bone surface (e.g., thickness of bone, cross-sectional shape, etc) are not taken into consideration.
Therefore, if two measurement targets mutually compared are different in the thickness of bone, their measurement results cannot be directly compared. As a result, evaluating the cured state of a fractured bone may fail if distortion data of a healthy bone having the same thickness is unavailable.
In view of the above circumstances, the present invention provides an ultrasound diagnostic apparatus capable of appropriately evaluating dynamic characteristics of a bone.